The present invention relates to fluids useful for subterranean operations, and more particularly, to mixed surfactants, treatment fluids comprising mixed surfactants, and methods of use employing such mixed surfactants to treat formations comprising multiple rocks.
Various procedures have been developed and utilized that may increase the flow of hydrocarbons from hydrocarbon-containing subterranean formations penetrated by well bores. For example, a conventional production stimulation technique may involve creating and extending fractures in the subterranean formation to provide flow channels therein through which hydrocarbons flow from the formation to the well bore. The fractures may be created by introducing a fracturing fluid into the formation at a rate sufficient to exert a sufficient pressure on the formation to create and extend fractures therein. Solid fracture proppant materials, such as sand, may be suspended in the fracturing fluid so that upon introducing the fracturing fluid into the formation and creating and extending fractures therein, the proppant material may be carried into the fractures and deposited therein. Such a treatment may prevent the fractures from closing due to subterranean forces when the introduction of the fracturing fluid has ceased.
Water-based fluids are typically used to fracture the subterranean formation, and when treated with conventional surfactants to recover the fracturing fluid, typically only a small portion of the fracturing fluid can be recovered. The aqueous fluid retained in the formation may increase the water saturation level of the formation, adversely affecting, among other things, the relative permeability to hydrocarbon flow, effective flow area, fracture length and the well productivity.
The water saturation levels of the formation may also increase due to, among other things, cross flow from water-bearing zones or other regions of the formation and filtrate invasion from water-based drilling fluids. In the latter case, the water saturation level of the formation near the well bore may be especially high, which may lower the relative permeability of the formation and thus the production of hydrocarbons by the water-blocked formation.
In gas wells, besides water blocks, liquid hydrocarbons may accumulate and lower the permeability of the formation rocks. Moreover, liquid hydrocarbons that condense out of the gas phase due to the decline in pressure below the dew point pressure of the gas also may hinder the production of hydrocarbons.
Several conventional surfactants have been used in attempts to alleviate these problems. Surfactants, which contain a hydrophilic and a hydrophobic group, are mixed with a treatment fluid, inter alia, to lower the surface tension of the fluid in order to facilitate the cleanup and mitigate formation damage caused by either water blocks or gas condensates. In addition to lowering surface tension, surfactants also may change the formation wettability. This results from a decrease in the capillary pressure of the flow channels in the subterranean formation, which may be accomplished by, among other things, changing the contact angle so that clean-up process can be very facile and hydrocarbon can flow with less resistance.
Cationic, anionic and zwitterionic surfactants may be used to enhance the production of hydrocarbons. While these surfactants have been used successfully, there may be disadvantages. Formations are commonly made up of various rocks. In general, different types of surfactants provide better treatment for each type of rock. For example, anionic surfactants are better for limestone formations and cationic surfactants are better for sandstone formations. Using a mixture of surfactants to treat the various rock may be difficult since at least some portion of the surfactants may precipitate with each other inadvertently, which may hinder or prevent optimal production of hydrocarbons. In addition, anionic surfactants on their own can also be sensitive to precipitation with multivalent cations further preventing optimal hydrocarbon production.